Rapid, competitive radium uptake in strontium, barium, and lead sulfates during sulfuric acid leaching

Abstract

Uranium- and thorium-bearing base metal mineral deposits contain daughter radionuclides which must be monitored and preferably removed or reduced during the process of generating base metal sulfide concentrates. Understanding the behavior of these radionuclides (focusing on 226Ra in this study) is critical for minimizing their concentrations in final economic products. To this end, Ra uptake into Sr, Ba, and Pb sulfates was evaluated experimentally under various conditions, including those approximating processing plant environments. Lead activity was also monitored, as 210Pb is also a radionuclide of concern. To simplify experiments, synthetic crystals of celestine (SrSO4), baryte (BaSO4), and anglesite (PbSO4) were grown in silica gel and subsequently exposed to RaCl2 solution at both low and neutral pH, for both 40 and 210 h. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) transects were performed across the grains to determine uptake of Ra (as well as trace Sr, Ba, and Pb) in the sulfates. Results indicate that Ra uptake in celestine is efficient when Ba and Pb are absent but is reduced to nearly zero when competing with baryte and anglesite. High acid sulfate activity inhibits uptake. Baryte incorporates significant Ra under all conditions. Anglesite is affected by coupled dissolution-reprecipitation mechanisms, resulting in dissolution of PbSO4 followed by precipitation of a mixed (Pb,Ba,Sr,Ra)SO4 phase in its place. Again, high sulfate activity inhibits this reaction. With this knowledge, it may be possible for process engineers to purposefully stimulate precipitation of Ra (and possibly Pb) onto a sulfate matrix, given the right conditions. Precipitation of RaSO4 (and 210PbSO4) onto a removable phase during processing would result in sulfide concentrates with natural background concentrations of radionuclides. Results from this study, including semi-quantitative Ra concentration data obtained via in situ LA-ICP-MS analysis add to data pertinent to management of Ra in boiler scales, oil and gas pipelines, environmental remediation, nuclear medicine, nuclear fuel processing and waste storage, among other industrial and research applications.